1. Technical Field
This invention relates to optical information media such as read-only optical disks and optical recording disks, and a method for evaluating the mar resistance of the same.
2. Background Art
To accommodate a vast quantity of information as typified by moving image data, advanced optical information media are required to increase their recording density. Active research and development works have been made to achieve a higher density for increasing the recording capacity. Among such research works, one proposal relating to digital versatile disks (DVD) is to shorten the wavelength of recording/reading light and increase the numerical aperture (NA) of an objective lens, thereby reducing the focused spot diameter of recording/reading light. As compared with compact disks (CD), DVD is successful in achieving a recording capacity of 6 to 8 folds (typically 4.7 GB/side) by changing the recording/reading wavelength xcex from 780 nm to 650 nm and the numerical aperture from 0.45 to 0.60.
For long-term recording of moving images of quality, an attempt was recently made to achieve a recording capacity of at least 4 folds of that of DVD by reducing the recording/reading wavelength to about 400 nm and increasing the numerical aperture to about 0.85.
However, several problems arise in establishing such a recording/reading system having increased NA. One exemplary problem is a reduction of the tolerance for the tilt of the information recording layer of the medium. More particularly, as is well known in the art, the tilt margin, that is a permissible tilt of the information recording layer relative to incident light, is in proportion to xcex/[txc3x97(NA)3] wherein xcex denotes the wavelength of recording/reading light and xe2x80x9ctxe2x80x9d denotes the thickness of a substrate. The tilt margin dramatically declines as the NA increases.
Also if the optical recording medium is inclined or tilted, a wavefront aberration (or coma) occurs. The coefficient of wavefront aberration W is represented by the formula I below.
W=(xc2xd)xc3x97txc3x97{n2xc3x97sin xcex8xc3x97cos xcex8}xc3x97NA3/(n2xe2x88x92sin 2xcex8)xe2x88x925/2xe2x80x83xe2x80x83Formula I
In formula I, n denotes the refractive index of the transparent substrate (referred herein as light-transparent substrate) by which a recording/reading laser beam is transmitted before reaching the information recording layer and xcex8 is a tilt angle. It is appreciated from formula I that the thickness of the light-transparent substrate must be reduced in order to acquire a certain tilt margin.
For this reason, the DVD is given a tilt margin by reducing the thickness of the light-transparent substrate to about one half (about 0.6 mm) of the thickness (about 1.2 mm) of the conventional CD substrate. For the system with a NA equal to 0.85, the thickness of the light-transparent substrate is reduced to about 0.1 mm.
Other problems associated with the increased NA are reduced focal depth and reduced working distance between the light-transparent substrate surface and the objective lens. The focal depth decreases in inverse proportion to (NA)2 and so, the focusing servo system is likely to become unstable and consequently, very sensitive to mechanical precision, flaws and stains on the light-transparent substrate surface. Moreover, the working distance decreases as the NA increases, provided that the objective lens diameter is fixed, with the increased risk of collision of the pickup casing or objective lens against the light-transparent substrate surface. For example, the system with NA=0.85 will have a focal depth of xc2x10.3 xcexcm and a working distance of about 100 to 300 xcexcm so that the focusing servo system is likely to become very unstable.
Also, objective lenses are generally made of plastics and glass, and it is a common practice to provide a protective plate around the lens as a precaution to the possible contact of the lens with the light-transparent substrate surface. In the case of a plastic lens, a protective plate known as edge guard is integrally molded for the protection of the lens surface. In the case of a glass lens, a plate of plastic material such as acrylic resin or polypropylene is attached around the lens as a lens protector. The general design is such that even if the pickup contacts the disk, the protective plate portion comes in preferential contact to prevent the objective lens surface from being flawed.
Meanwhile, most optical disks now available in the market use rigid substrates of thermoplastic resins such as polycarbonate and polymethyl methacrylate as the light-transparent substrate. This means that data are recorded and reproduced with a laser beam which enters the information recording layer past the surface of these resinous substrates. These resins are optically homogeneous, highly transparent, easily moldable and excellent in mechanical strength, but have drawbacks including a low surface hardness and mar susceptibility. It is then a common practice to provide the resinous substrate on its surface with any hard coat layer as the mar-proof layer. In forming such hard coat layers, it is most customary to apply a polymerization curable compound having at least two polymerizable functional groups such as acryloyl groups in a molecule to the substrate surface, and irradiating thereto actinic radiation such as UV radiation for curing for thereby forming a hard coat layer.
However, these resins of the UV curing type have a certain limit of surface hardness achievable, despite superior abrasion resistance as compared with thermoplastic resins including polycarbonate, and do not always provide fully satisfactory abrasion resistance for use as optical information media. The system in which a high density is achieved by increasing the NA of the recording/reading optical system requires that disks be protected from not only damages during the user""s handling of the disk, but also damages by the above-mentioned collision of the pickup. Accordingly, there is a need for a hard coat layer which has a dramatically improved hardness as compared with the aforementioned UV-curable resins.
To solve such a problem, methods for forming hard coat layers based on inorganic compounds have been proposed.
For example, JP-A 11-203726 discloses a method of forming a hard coat layer on an optical disk having a recording layer and a light-transparent layer successively stacked on a substrate, wherein recording and/or reading is performed with incident light from the light-transparent layer side, the method involving the steps of depositing inorganic materials such as SiN and SiO on the light-transparent layer surface as two or more layers by an ion beam sputtering or similar technique, the deposited layers serving as a protective film or hard coat layer. However, the inorganic film formed by such a technique as sputtering or evaporation has increased internal stresses. Then, when the film is grown to a thickness of more than several hundreds of nanometers in order to establish satisfactory abrasion resistance, the film becomes self-destructible. Then the inorganic film of this type is difficult to provide a hard coat layer having substantially satisfactory abrasion resistance.
JP-A 8-263878 proposes a method of forming a silica-based thin film on a substrate surface by a sol-gel process using a solution of alkoxysilane or the like, the thin film serving as a protective film or hard coat layer. When the inorganic compound film is formed by the sol-gel process, however, baking at a high temperature in extreme excess of 100xc2x0 C. is necessary to promote the reaction to a full extent to form a consolidated film. It is then difficult to apply this method to the surface of light-transparent substrates made of less heat resistant resins.
JP-A 2000-132865 describes an information recording medium having a protective layer made of polysilazane. The subject matter of this patent publication is an information recording medium having a disk or tape-shaped support, although the publication lacks an example in which the invention is applied to optical disks.
While the surface hardness serves as an index of the abrasion resistance of resinous materials or inorganic materials used as the hard coat layer, it is generally measured as indentation hardness (as typified by Vickers hardness) or scratch hardness, or by an abrasion test. Of these measuring methods, in the abrasion test, for example, the abrasion loss of a test specimen resulting from abrasion is often quantitatively determined using changes of several parameters such as the weight, thickness and light transmittance of the test specimen. For optically transparent materials having a relatively high surface hardness like the hard coat layer materials for optical information media, it is most adequate to quantitatively determine the hardness using a change of light transmittance or light diffusion. Specifically, it is customary to measure the haze of the test specimen on which white parallel light is incident.
The method of evaluating the hardness of a specimen utilizing haze measurement is effective as a method of quantitatively determining the visual deterioration of a specimen caused by abrasion. However, this evaluation method offers macroscopic evaluation using incoherent light, that is, non-convergent light, which is not always correlated to the degree of deterioration of recording/reading characteristics of the optical information medium caused by abrasion. Therefore, the above method is not regarded as an appropriate method of evaluating the performance of a hard coat layer on an optical information medium. Additionally, the evaluation method based on haze measurement is to measure the light transmitted by a transparent specimen, and so, in evaluating a hard coat layer on an optical information medium, a self-supporting film equivalent to the hard coat layer must be previously furnished before measurement can be made. On the other hand, the abrasion resistance of the hard coat layer depends on the surface property, coefficient of friction, and thickness of the hard coat layer and the modulus of elasticity of the material of which the hard coat layer is made although the factors governing the abrasion resistance are not limited thereto. For example, the hardness of the substrate on which the hard coat layer is formed also has a substantial influence on the abrasion resistance of the hard coat layer at its surface. Accordingly, in this regard too, for accurate evaluation of the performance of a hard coat layer, it is preferred to evaluate the hard coat layer as actually formed on an optical information medium.
Heretofore, there has not been available a method of evaluating or inspecting a hard coat layer while meeting various requirements as mentioned above, and it has been impossible to ascertain the accurate performance of a hard coat layer. However, an optical disk system having a capacity increased due to the reduced wavelength and increased NA of a recording/reading optical system, which is now under investigation for practical use, is more sensitive to flaw and stain on the surface of a light-transparent substrate than the existing CDs and DVDs. Accordingly, the requirements on the abrasion resistance and mar resistance of the hard coat layer become more strict than in the prior art, and there is a strong desire to have a method suitable for the quantitative determination of these properties.
Also where the light-transparent substrate as a whole is constructed of a high hardness material rather than the provision of a hard coat layer on the surface of a light-transparent substrate, the mar resistance of the light-transparent substrate can be altered by such means as forming a recording layer or joining together with another substrate. Accordingly, in this case too, there is a desire to have a method capable of evaluating the mar resistance of a light-transparent substrate under the same conditions as in actual media.
The present invention has been devised under the above-mentioned circumstances, and its object is to provide an optical information medium which is less susceptible to mar or flaw during the user""s handling, and specifically, an optical information medium having a recording density increased due to the increased NA of a recording/reading optical system, which medium is less susceptible to mar or flaw by collision of a pickup during the recording and/or reading operation. Another object of the invention is to provide a method for evaluating the mar resistance of an optical information medium on its recording/reading light incident side surface, by quantitative determination in a state reflecting an actual service environment and in a simple way.
The above objects are attained by the present invention which is defined as (1) to (11) below.
(1) An optical information medium comprising a light-transparent substrate and an information recording layer, wherein optical recording and/or reading is performed with a laser beam that enters said information recording layer from the light-transparent substrate side, wherein
said medium includes a cured polysilazane film disposed on the laser beam incident side of said light-transparent substrate, and
said light-transparent substrate or said light-transparent substrate having the cured polysilazane film integrated thereon has a tensile modulus of at least 200 MPa.
(2) The optical information medium of (1) wherein said light-transparent substrate includes a resin layer of 30 to 300 xcexcm thick.
(3) The optical information medium of (1) or (2) wherein said cured polysilazane film has a thickness of 0.2 to 50 xcexcm.
(4) The optical information medium of any one of (1) to (3) wherein the laser beam incident side surface has a pencil hardness of at least HB.
(5) The optical information medium of any one of (1) to (4) wherein said cured polysilazane film is a laminate including a plurality of films of different compositions, in which a cured film of inorganic polysilazane and a cured film of organic group-introduced polysilazane are stacked in the described order when said cured polysilazane film is viewed from the laser beam incident side.
(6) The optical information medium of any one of (1) to (5) further comprising a functional layer on the laser beam incident side of said cured polysilazane film, said functional layer having at least one function selected from among lubricity, water repellency and oil repellency.
(7) The optical information medium of (6) wherein said functional layer has a thickness of up to 500 nm.
(8) The optical information medium of (6) or (7) wherein said functional layer is made of a compound having hydrolyzable silyl groups.
(9) In connection with an optical information medium comprising a light-transparent substrate and an information recording layer, wherein optical recording and/or reading is performed with a laser beam that enters said information recording layer from the light-transparent substrate side,
a method for evaluating the optical information medium for mar resistance on its laser beam incident side surface, comprising the steps of intentionally abrading the laser beam incident side surface of the optical information medium; then measuring a recording/reading characteristic; and evaluating the mar resistance of the laser beam incident side surface on a basis of the measured value.
(10) The method of (9) wherein an abrasive wheel as prescribed by ISO 9352 is used as the means for intentionally abrading the laser beam incident side surface of the optical information medium.
(11) The method of (9) wherein #0000 steel wool is used as the means for intentionally abrading the laser beam incident side surface of the optical information medium.
Searching for a hard coat layer suitable for protecting the surface of a light-transparent substrate in an optical information medium, we have found that a film formed by curing polysilazane is appropriate for the purpose. Use of a cured polysilazane film as a protective layer on information media is described in the above-referred JP-A 2000-132865. It is noted that the Examples demonstrated therein are only those media in which a red sensitive layer of 0.2 xcexcm thick containing a dye and a resin binder is formed on a polyethylene terephthalate support of 20 xcexcm thick and a protective layer of polysilazane and 0.1 xcexcm thick is formed on the red sensitive layer. It is described therein that the protective layer preferably has a dry film thickness of 0.001 to 0.2 xcexcm, and more preferably 0.05 to 0.15 xcexcm. The reason why the thickness of the protective layer is limited to this range is described nowhere.
In Examples demonstrated in the above publication, tape media are evaluated in the state accommodated in a cartridge. Since the tape medium does not contact any member other than the cartridge during handling, there is little chance for the tape medium to be marred. The above publication pays no attention to the collision of a pickup, which becomes a problem for optical disks. By contrast, most optical disks are of the type that disks are not accommodated in cartridges, and they are susceptible to mars and flaws on their laser beam incident side surface. Also, those optical disks applied to the recording/reading system with increased NA have an increased chance for a pickup to collide against the laser beam incident side surface. Therefore, the optical disk on its laser beam incident side surface is required to have improved abrasion resistance and mar resistance. Although the cured polysilazane film has a high hardness, the thickness of 0.1 xcexcm described in the above-referred JP-A 2000-132865 is insufficient to provide the abrasion resistance and mar resistance required for the optical disk on its laser beam incident side surface.
However, it was empirically found that since the cured polysilazane film has increased internal stresses, forming the cured polysilazane film to an increased thickness invites a likelihood of self-destruction. Continuing experiments in order to prevent the cured polysilazane film from self-destruction, we have found that the likelihood of self-destruction depends on the tensile modulus of a light-transparent substrate underlying the cured polysilazane film. Then the present invention sets the tensile modulus of the light-transparent substrate in a specific range for making it possible to form the cured polysilazane film to a substantial thickness, eventually obtaining an optical information medium having satisfactory abrasion resistance and mar resistance on the laser beam incident side surface.
The evaluation method of the invention can evaluate the mar resistance of an optical information medium on its laser beam incident side surface, by quantitative determination in a state reflecting an actual service environment and in a simple way.